The present invention relates to the storage of hydrogen, and discloses a structure which can be used in many practical applications. Certain metal alloys can react with hydrogen to form solid state metal hydrides, wherein the density of hydrogen is greater than in liquid hydrogen. This general concept is explained in a paper entitled "How Metals Store Hydrogen", Chemtech, volume 11, pages 754-62 (1981).
The hydrogen pressure needed to induce a metal to form a hydride, or the pressure of hydrogen evolved from a metal hydride, depends on the temperature. By cycling the operating temperature, a metal hydride can be induced to give up gaseous hydrogen at a higher pressure than the pressure at which the hydrogen was originally absorbed.
The properties of metal hydrides, described briefly above, form the basis for several applications of commercial interest. These include a means for storing hydrogen at high density and low pressure, a hydrogen heat pump, a hydrogen compressor having virtually no moving parts, and a means for separating hydrogen from other gases.
One of the problems in the use of metal hydrides as described above is the need to achieve rapid heat transfer. The reaction wherein hydrogen is added to a metal to form a metal hydride is quite exothermic. Thus, when hydrogen is being converted from gaseous to hydride form, there must be a means for rapidly conducting a large amount of heat away from the apparatus. Conversely, when it is desired to liberate hydrogen gas from the hydride, it is necessary to direct a large amount of heat into the hydride. Hydrogen storage devices of the prior art have been limited by their capacity for rapid addition or removal of heat.
Examples of prior art devices for storing and recovering hydrogen using metal hydrides are shown in U.S. Pat. Nos. 4,396,114 and 4,402,187. The cited patents disclose a structure wherein the hydride is contained inside a heat transfer tube, in the annular space surrounding a central porous tube through which hydrogen is charged or discharged. The hydride is disposed in a layer about 2-3 mm thick. This layer is thinner than in any known prior art hydrogen storage system, but is not as thin as in the system of the present invention. A thin layer is essential because a bed of granules has low thermal conductivity.
The present invention discloses a structure which permits very rapid heat transfer to and from a metal or metal hydride. The invention also includes several practical applications, which are feasible only because of the use of the unique structure disclosed herein. The invention also includes a method for applying a metal or metal alloy, capable of forming a metal hydride, to a metal surface.